PvE or sPvP? Also there already is one thread about thief builds and a general thief discussion thread. However, here is my backstab build. You're missing a lot of Might stacks and damage, also Assassin's Signet.

I like your build, I just feel that it lacks a certain amount of mobilty to better position backstabs and initi regen for CnD, which aids both damage in the long run and survivability due to more time stealthed.